Generally speaking, this invention relates to electromechanical transducers for converting mechanical movements or displacements into electrical signals. More particularly, this invention relates to an improved strain sensitive element or force gage for use in such mechanical transducers.
In electromechanical transducers of the kind to which the present invention is directed, a transducing element is utilized for detecting the relative displacement of two parts and for developing a corresponding electric signal. Generally, such relative displacements have been measured in the past with various kinds of strain gages. However, these have a tendency to be of considerable weight, some of which are very bulky, some of which are not very sensitive. Those that are have intricate designs which are very expensive. As mentioned above, the present invention is directed to a force-type sensor or gage which is mounted between two parts between which a force is applied. The gage is, therefore, strained in an amount which depends upon that force. It is substantially smaller than prior art force gages, is relatively simple in structure, is easily manufactured, and is, therefore, less expensive.
As such piezoresistive transducers have developed in use over the years, it has become increasingly desirable to have extremely small sensors of high sensitivity and low bulk. However, in order to develop force gages which are of extremely small size, difficulties arise in the handling thereof for subsequent mounting upon their substrate, once they are developed. They are difficult to handle not only because of their small size, but also because of their fragility.
One of the primary advantages of force transducers lies in the fact that the displacement between the pads at each end thereof produced by relative motion of the two parts to which the pads are attached is concentrated in the "suspended", so to speak, portion of the force gage which can mechanically amplify the strain being sensed or measured. Furthermore, the resistance change of the element per unit displacement is greatest as the length of the element is reduced. By use of both short gage lengths and appropriate leverage very large resistance changes may result from very small displacements. This change in resistance is determined by means of electrical current flowing through the element from one pad to the other, and measuring changes in voltage or other electrical properties resulting from changes in resistance. However, when attempts are made to reduce to a smaller size such force gages, then, as mentioned above, difficulties arise relative to the handling thereof in mounting upon their substrates, as well as other problems which ordinarily arise in handling very small objects.
With this invention, by contrast, strain sensitive elements are provided in the form of force gages which are derived from the substrate upon which they are subsequently supported in use. That is, the gages are defined upon the substrate or marked thereon, and subsequently etched right from the material of the substrate. In one form of force gage of the invention, the gage is etched to allow a small support or mesa underneath, while maintaining the gage still connected by this minute portion of the substrate to the substrate proper. In its preferred form, the invention is directed to a force gage which is etched free of its substrate along its length but continuous with it at its ends. Thus, the gages of the invention are crystallinally continuous with their support.
That is, force gages of substantilly smaller strain volume are produced by defining the gage in the substrate or in material rigidly bonded to the substrate, and subsequently etching away the immediately adjacent material, leaving the gage free in space, after the fashion of force gages of the past, but supported against unwanted cross loads by remote portions of the substrate. Such gages may have volume as small as 3.times.10.sup.-10 cubic centimeters of stressed material, as opposed to present commercially available force gages wherein the strained volume is 5.times.10.sup.-7 cubic centimeters. Both gages would typically be strained to one part per thousand. The strain energy is thus a thousandfold less for the smaller gage.
It will be appreciated, in this connection, that the volume of the gages formulated according to the invention here will vary widely depending upon ultimate use. For example, a "sturdy" gage may have 3.times.10.sup.-4 times 8.times.10.sup.-4 times 32.times.10.sup.-4 centimeters or 10.sup.-9 cubic centimeters. On the other hand delicate gage may have 0.3.times.10.sup.-4 times 3.times.10.sup.-4 times 12.times.10.sup.-4 centimeters, or 10.sup.-11 cubic centimeters. It is within the purview of this invention to obtain a gage volume of 10.sup.-12 cubic centimeters utilizing electron beam lithography.
In considering the conditions generally for carrying out the process of the invention here for producing a force gage, a conventional silicon crystal material is selected, and the outline of the gage is etched on the selected crystal which forms the substrate. An etch is selected which is both anisotropic and doping-selective. Caustic, hydrazine, and pyrocatechol etchants may be selected, depending upon the results desired. They attack silicon rapidly in the [112] direction, moderately rapidly in the [110] direction, and very slowly in the [111] direction. With this invention, the substrate orientation is (110) plane and [111] along the gage so as to define a groove over which the gage extends. With such orientation, a groove is produced with walls which are nearly vertical, and with floors that are nearly flat.
The same etchants which are anisotropic are dopant selective, in that they attack very slowly silicon in which a boron concentration is developed which is greater than 5.times.10.sup.19 /cc. In accordance with the process of the invention, the gage is defined and its terminals are also defined by a planar diffusion or ion implantation through an oxide mask to a boron concentration of roughly 10.sup.20 /cc. The boron makes the gage P-type, while the substrate is N-type. The diffused area is electrically isolated from the substrate by a P-N junction. During the etching procedure which forms the groove, the gage is exposed to the etchant, but is resistant to it. As will be appreciated, and explained further herein, when the groove is defined over which the gage extends, a hinge is also defined in the substrate around which one end of the substrate moves relative to the other to develop the strain being monitored by the sensor. Also, the hinge protects the gage against transverse loads.
As a further feature of the invention, two substrate wafers may be bonded together. Grooves may be formed either before or after bonding of the wafers. If the groove is formed by impact grinding, it must be formed before bonding. Gages and their terminals may be defined in the gage wafer by doping them to the requisite high concentration of boron before bonding the wafers, then etching away all of the undoped portion of the gage wafer. Alternatively, the whole bonded surface of the gate wafer may be doped with boron so that the etching leaves a continuous sheet of gage material from which gages may be etched by a subsequent photolithographic step. This is similar to the bonded wafer approach described and claimed in co-pending U.S. application Ser. No. 233,728, filed Feb. 12, 1981, now U.S. Pat. No. 4,400,869 issued Aug. 30, 1983 which application is owned by the assignee of this application and which application is incorporated by reference herein in its entirety.
For example, the gage wafer will still be (110)[111], while the hinge wafer is (100)[110] for easy and precise etching. This gives less difference in strain on the gages and the associated hinge surface than does the square etch pattern into (110). Once the two wafers are bonded together, with the gages positioned over their appropriate grooves or apertures which have been defined in the wafers, then the gages are freed by etching away all of the gage wafer except the gages and their terminals. This approach is more complex in its execution, but offers dialectric isolation of the gages, rather than diode isolation. Also, this allows the use of different crystal orientations in the gage and substrate wafers. Of course, this approach departs from one of the primary aspects of this invention which is having the crystal structure of the gage the same as its substrate support.
A piezoresistive transducer developed in accordance with the general procedures noted above is particularly appropriate for use in accelerometers, pressure transducers, and displacement gages. The length of each individual gage produced in accordance herewith, will be generally about 25 microns, while the width will be about 6 microns.
The general steps or procedure involved in fabricating a piezoresistive transducer dice for use in an accelerometer includes first selecting a silicon wafer. In this connection, it should be understood that a plurality of sensors are produced in a single wafer depending upon the form of sensor being developed in any particular application. Subsequently, the individual sensors are diced out of the wafer, once the sensors have been formed with their gages, in accordance with this invention. After the wafer is selected, it is heavily oxidized. Subsequently, index marks are imposed on either side of the wafer photolithographically in order to align the patterns on each side of the wafer. It should be pointed out here, that with respect to each die formed on a wafer, gages may be formed on one or both sides of the wafer, again depending upon the form of sensor being developed for a particular application.
Subsequent to imposing the index marks on each side by photolithographic means, apertures are opened in the oxide layer which are to be heavily doped to define the gages and conductors therefore. After this is done, boron is implanted into the open areas on both sides in the amount of 1.5.times.10.sup.16 cm.sup.2, sufficient to obtain boron in the amount of at least 5.times.10.sup.19 atoms per cubic centimeter, and a depth within the range of between about 0.1 and 3 micrometers. The implantation should provide nearly equal doping on both sides. Subsequent to the implantation of the boron, the silicon wafer is annealed at a temperature of 920.degree. C. for about one hour. In this connection, for a more detailed discussion about general procedures of the kind carried out and discussed here, reference is made to the above noted co-pending U.S. application Ser. No. 233,728. The same boron doping can be achieved by planar diffusion.
Once the annealing procedure has taken place, the etching patterns are opened on both sides photolithographically. Thus, the wafer is prepared for the etching procedure. Etching may be done by a potassium hydroxide-water-isopropyl alcohol bath. Preferably, however, an ethylene diamine-pyrocatechol etch is utilized. In this connection, during this etching procedure, areas protected by oxide and areas heavily doped with boron do not etch. The etching procedure takes approximately four hours. Preferably, etching is to a depth of about 0.0022 inches assuming a wafer of 0.005 inches to leave a central hinge of 0.0006 inches. The depth should be sufficient to obtain a substantially level bottom surface of the groove below the gages. Also, depth should be sufficient that residual thickness at the bottom of the groove, considered as an elastic hinge, represents a small fraction of the bending stiffness in a system consisting of the formed hinge and its gage.
Once the etching procedure has taken place, all of the previously applied oxide is stripped and a thin oxide layer is grown on the wafer to protect the P-N junctions. Once this has taken place, aluminum is deposited on one or both sides to provide the metallic connections for the individual gage or gages. In this connection, once the aluminum has been deposited, then the patterns of the aluminum for forming the contact areas are photolithographically defined on the wafer. Subsequently, the wafer is cut into the individual dice with a diamond saw.
With the foregoing and additional objects in view, this invention will now be described in more detail, and other objects and advantages hereof will be apparent from the following description, the accompanying drawings, and the appended claims.